How many molecules are there in 34.78 g of potassium phosphate?

Potassium phophate has the formula K3PO4. To find the number of molecules in 34.78 grams of K3PO4, you must find its molar mass and then the number of moles. 

To find K3PO4's molar mass, add up all of the molar masses of the elements of the molecule:

                         K              P               O

Molar mass = 3(39.1 g) + 31.0 g + 4(16.0 g) = 212.3 grams per mole

34.78 grams K3PO4 x (1 mole/212.3 grams) = 0.1638 moles K3PO4

To convert from moles to molecules, use Avogadro's number:

0.1638 moles K3PO4 x (6.023 x 10^23 molecules/mole) = 9.867x10^22 molecules

Conversion factors can be written in two forms. For example, two forms of the relationship between mass and moles of K3PO4 are:

212.3 grams/1 mole and 1 mole/212.3 grams

The correct form of a conversion factor to use is the one that has the unit you're converting to in the numerator and the unit you're converting from in the denominator so it cancels out.